Just a different data point; I've been on it when there was what I'd estimate as a 5 year old and the poor guy screamed in panic the whole ride... Just know your kid, as some of them would eat it up, some would hate it.
As for "jerky" I'd say yes, I find it a lot more jerky than FoP, for instance. But, our family younglings always enjoyed it, as well as us oldlings. lol
My kids have always loved Star Tours.
I find it jerky. The movement plus the 3D makes it the only attraction at WDW that makes me motion sick. I have to close my eyes the whole time. My kids love it, so I do it.
You do get bounced around, especially if you get the one scene where you go underwater. There's a "creature" (sorry not a Star Wars guy) that latches onto you and shakes your vehicle side to side. I felt tossed around more in Body Wars than Star Tours but your mileage may vary.
We were on this ride many...MANY...times this past week (my son is a huge SW fan) and I have to say that it depends on which "tour" you get. Some were pretty rough and some were milder. If your child is familiar with Star Wars...it should not be very scary for them.